4.3 SPECIFIC LATENT HEAT

CHAPTER 4
HEAT

4.3 SPECIFIC LATENT HEAT

TEXT BOOK : PAGE 137 - 147

SUE ROSE

4.3 SPECIFIC LATENT HEAT

LEARNING

STANDARD 1

4.3.1 Explain latent
heat

The ‘ais kacang’ which is colder than
surrounding is absorbing heat from
surrounding air.
What changes happens to the ‘ais
kacang’?

A fish can be cooked by steaming in a
steamer pot.
What changes happens to the water
in the steamer pot when it boiled?

Elements can exist in three states: solid, liquid and gas

SOLID LIQUID GAS

Stronger molecular Less strong molecular The bond between gas
bonds in solid bonds in liquid molecules is the
weakest – gas
molecules moves
freely at random.

Activity 1 Aim: To study the change in temperature when heat is supplied to
a solid at its melting point.

Heat the ice in a beaker slowly.

(a) Do the termometer reading increase?

The temperature remain the same

(b) What is the effect of the heat supplied to
the ice?

• Heat is absorbed by ice is used
to break up the bonds between
ice molecules.

Activity 1 Aim: To study the change in temperature when heat is supplied to
a solid at its melting point.

Heat the water in a beaker. What is the effect of the heat to water?

The temperature of water rises up until 100 ⁰C

Continue to heat even though the water in the Latent heat
beaker starts to boil.
(a) Is the temperature rise up?

The temperature remain the same.

(b) If not, what is the effect of the heat
supplied to the boiling water?

To break up the bonds between the
water molecules to change to gas

Name the heat absorbed or given out at
constant temperature during change of phase

Changes in phase of matter

During the changes in the phase of matter such as melting and boiling, the
temperature remains constant even though heat is supplied continually.

LATENT Heat that is absorbed during melting and boiling
HEAT without change in temperature.

The heat absorbed or the heat
released at constant temperature
during change of phase.

4 Main Changes of Phase

MELTING : BOILING:
Latent heat is absorbed Latent heat is absorbed

FREEZING : CONDENSATION:
Latent heat is released Latent heat is released

Heat absorbed for Latent Heat of Fusion Heat absorbed for
a substance to a substance to
LATENT
change from solid HEAT change from liquid
to liquid at to gas at constant
constant
temperature temperature

Latent Heat of Vaporisation

Why the temperature remains constant during a change of phase?

• Heat absorbed is used to break up the bonds between
solid molecules.

• The kinetic energy of the molecules does not increase.
• Temperature is directly proportional to kinetic energy.
• Therefore temperature remains constant when ice melt

to water.

When a liquid boils, the heat absorbed is used to:

Completely break up the bonds between particles.

Do work against atmospheric pressure when the
gaseous vapour expands into the atmosphere

LEARNING STANDARD 2

4.3.2 Define:

(i) Specific latent

heat, l = Q/m

(ii) Specific latent heat

of fusion, lf

(iii) Specific latent heat

of vaporization, lv

Specific Latent Heat of Fusion

Heat absorbed for 1 kg substance to
change from solid to liquid at constant
temperature

SPECIFIC Heat absorbed for 1 kg substance to
LATENT HEAT change phase at constant temperature

Heat absorbed for 1 kg substance to
change from liquid to gas at constant

temperature

Specific Latent Heat of Vaporisation

Heat J
kg
Mass J kg-1

Specific Latent
Heat

Specific Latent Heat of Fusion

Boiling

Specific Latent Heat of Vaporisation

What does it means by the specific What does it mean by the specific
latent heat of fusion of ice is latent heat of vaporization is
336 000 J kg-1? 2 260 000 J kg-1?

336 000 J of heat is 2.26 x 10 6 J of heat is
needed for 1 kg ice to melt needed for 1 kg of water to
to become water at 0 °C. boil to become gas at 100 ⁰C.

• Heat absorbed • Heat absorbed • Heat absorbed
• Temperature increases • Temperature increases • Temperature
• Kinetic energy molecules of • Kinetic energy of
increases
increases molecules increases • Kinetic energy of
• L: Solid M: Liquid • N: Liquid O: Gas
molecules increases

• Heat absorbed
• Constant

temperature
• Kinetic energy of

molecules constant
• M: liquid MN: Liquid

& gas N: Gas

• Heat absorbed
• Constant temperature
• Kinetic energy of molecules constant
• M: Solid MN: Solid & Liquid
• N: Liquid

• Heat released • Heat released • Heat released
• Temperature decreases • Temperature decreases • Temperature
• Kinetic energy of molecules • Kinetic energy of
decreases
decreases P: Gas Q: Gas molecules decreases • Kinetic energy of
• R: Liquid S: Liquid
molecules decrease

• Heat released • Heat released
• Constant temperature • Constant temperature
• Kinetic energy of molecules constant • Kinetic energy of molecules constant
• Q: Gas QR: Gas & liquid • S: liquid MN: Liquid & solid
• R: liquid • T: Solid

Activity 3 Aim: To compare and discuss
(i) Specific latent heat of fusion of ice and wax
(ii) Specific latent heat of vaporization of water and oil

(a) Compare the specific latent heat of fusion of ice and wax.

Ice > Wax

(b) State the difference between ice and wax in terms of strength
of bond between molecules.

The arrangement between ice molecules are closer. The
strength of bond between ice molecules is stronger.

Activity 3 Aim: To compare and discuss
(i) Specific latent heat of vaporization of water and oil

(a) Compare the specific
latent heat of
vaporization for water
and petrol

Water > petrol

(b) State the difference The distance of separation between
between water and petrol molecules in water are closer. The
in terms of strength of strength of bond between water
bond between molecules molecules are stronger.
and distance of separation
between molecules in
gaseous phase

Specific latent heat of fusion of ice is 3.34 x 105 Jkg-1.
Specific latent heat of vaporization of water is 2.26 x 106 Jkg-1.
The specific latent heat of fusion of ice is 3.34 x 105 Jkg-1.
The specific latent heat of vaporization of water is 2.26 x 106 Jkg-1.
Why are the values different?

• In latent heat of fusion, the energy is given to
the molecules in solid so that they can move
freely as in a liquid.

• In latent heat of vaporization, a large amount
of energy is needed to separate the liquid
molecules to gas molecules and overcome
the atmospheric pressure to allow them to
move around as gas molecules.

When the heat added or When the heat added or removed
changes the phase of an object at
removed changes the constant temperature, the heat is

temperature of an object, calculated using:

the heat is calculated: Q = ml

Q = mcθ

If heat is supplied by electric heater to change the
phase of a substance, the equation Q = ml can be
written as

Q = Pt = ml

P = power of the heater, unit in W
t = time, unit is seconds

LEARNING

STANDARD 5

4.3.5 Solve problems
involving latent
heat

Exercise 1:

1. If 2 kg of ice melts when 452 000 J of heat energy is
supplied to it, what is the specific latent heat of ice?

L = Q/m
= 452 000 J
2 kg
= 226 000 J kg-1

2. An immersion heater supplies heat at a rate of 50
J s-1 to an insulated vessel containing a liquid.
The liquid is brought to its boiling point and kept
boiling for 2 minutes, during which time the mass
of the liquid lost is 40 g. What is the value of the
specific latent heat of vaporization of the liquid?

m = 0.04 kg P = 50 W
t = 2 x 60 =120 s

L = Q/m = Pt/m
= (50)(120) / 0.04
= 150,000 J kg-1
= 1.5 x 105 J kg-1

3. Ice cubes of mass 50 g at 0 ⁰C is heated by a 50
W. The ice takes 5 minutes to melt completely.
What is its specific latent heat of fusion of ice?

m = 50 g = 0.05 kg
P = 50 W
t = 5 x 60 = 300 s

L = Q/m = Pt/m
= 50 x 300
0.05
= 300 000 J kg-1
= 3.0 x 105 J kg-1

4. An immersion heater rated at 500 W is fitted into a
large block of ice at 0 ⁰C. The specific latent heat
of fusion of ice is 3.34 x 105 J kg-1. How long
does it take to melt 1.5 kg of ice?

m = 1.5 kg
P = 500 W
L = 3.34 x 105 J kg-1

t = ml = 1.5 x 3.34 x 105
P 500

= 1 002 s

5. A 500 W electric heater is used to boil water.
What is the time required to reduce the mass of
water by 70 g after the water has reached its
boiling point? (Specific latent heat of
vaporization of water is 2.26 x 106 J kg-1)

m = 70 g = 0.07 kg
P = 500 W
L = 2.26 x 106 J kg-1

Pt = ml
t = (0.07)(2,260 000)

500
= 316.4 s

6. 0.5 kg of water of 30 ⁰C is heated until all it is converted to steam.
What is the amount of heat energy supplied to it?

Specific heat capacity of water = 4 200 J kg-1 ⁰C-1
Specific latent heat of vaporization = 2.26 x 106 J kg-1

0.5 kg water 0.5 kg water 0.5 kg steam
30 ⁰C 100 ⁰C 100 ⁰C

Q1 = mcθ Q2 = ml
= 0.5 x 4200 x 70 = 0.5 x 2260000
= 147 000 J
= 1 130 000 J

Total = Q1 = mcθ + Q2 = ml
heat = 0.5 x 4200 x 70 = 0.5 x 2260000

= 147 000 J = 1 130 000 J

Q = 1 277 000 J

6. 3.2 kg of water at 28 ⁰C is heated until half of it is changes to
steam at 100 ⁰C. How much heat energy needs to be supplied to
the water?

Specific heat capacity of water = 4 200 J kg-1 ⁰C-1

Specific latent heat of vaporization = 2.26 x 106 J kg-1

3.2 kg water 3.2 kg water 1.6 kg steam
28 ⁰C 100 ⁰C 100 ⁰C

Q1 = mcθ Q2 = ml
= 3.2 x 4200 x 72 = 1.6 x 2 260 000
= 967 680 J
= 3 616 000 J

Total = Q1 = mcθ + Q2 = ml
heat = 3.2 x 4200 x 72 = 1.6 x 2 260 000

= 967 680 J = 3 616 000 J

Q = 4 583 680 J

LEARNING

STANDARD 3

4.3.3 Experiment to
determine :

i. specific latent
heat of fusion of
ice

ii. specific latent
heat of
vaporization of
water

Experiment 4.3 Aim: To determine specific latent heat of fusion of ice

Apparatus: Immersion heater (12 V, 50 W), 2 beakers, retort stand and
clamps, power supply, 2 filter funnels and triple beam balance and ice.

1. Measure the masses of beaker 1 ad
beaker 2 and record as m1 and m2
respectively.

2. Arrange the apparatus as shown in the
diagram. At first, both the beakers are
not placed under the filter funnels.

3. Switch on the power supply of Set 1
and wait until water flows from the filter
funnels at a steady rate. Place beaker
1 and beaker 2 under respective
funnels and start the stopwatch.

4. Switch off the power supply after 5
minutes.

5. Remove the beakers. Measure the
masses of both beakers again and
record as m3 and m4 respectively.

50 g
52 g
108 g
78 g

36 x 5 x 60 = 10, 800 J
108 - 50 = 58 g
78 - 52 = 26 g

58 - 26 = 32 g
= 0.032 kg

L = Pt/m = 10,800/0.032 = 337 000 J kg-1

Discussion 2. Set 2 is known as control set.
Explain the necessity of control
1. Explain why the stop-watch does set in this experiment.
not start right after the heater is
switched on?

The ice will be waited until • The ice in set 2 melts because
it melts constantly it get heat only from the
surroundings.
3. Compare the value of specific
heat of fusion of water with the • This control set is to ensure
exact value. Explain the that the ice melts due to the
difference between the two values heat from the heater only
(if any).

• The exact value is 3.34 x 105 J/kg.
• The value from the experiment is higher.
• The ice also absorbs heat from the

surrounding, so more ice will melt.

Experiment 4.3 Aim: To determine specific latent heat
of vaporization of water

Apparatus: Immersion heater (1 000 W),
stopwatch, weighing balance and water.

1. When the water is boiling, record the initial reading
of the balance and start the stopwatch at the same
time.

2. After 5 minutes, record the reading of the balance.

685 g
565 g
5 x 60 = 300 s
1 000 W

685 – 565 = 120 g = 0.12 kg

L = Pt/m
= 1 000 x 300/0.12
= 2,500 000 J/kg

Discussion Standard value for specific
latent heat of vaporization
1. Is the value of specific latent heat of of water is 2,260,000 J/kg.
vaporization from this experiment
smaller or bigger than the actual
value? Give reason.

The value will be higher because the water
also absorb heat from surroundings which

contribute more steam to be released.

LEARNING

STANDARD 4

4.3.4 Communicate to
explain the
applications of
specific latent heat
in daily life

Activity 5 Aim: To show the evaporation causes cooling
Changes in phase of water

When ice melts, the ice When water boils, the water When water vapour
molecules absorb latent heat molecules absorb latent condenses, the water vapour
heat of vaporization molecules release latent heat
of fusion causing ice to causing water to change
change from solid to liquid. from liquid to gas. of vaporization causing
water vapour to change from

gas to liquid.

Absorption of latent heat during melting and evaporation can
be used to give the effect of cooling.

Latent heat released during condensation however
is used for the purpose of heating.

A. Material: beaker, alcohol, thermometer

1. Dip one of your finger into the alcohol.

2. Take out your finger and wait for the alcohol to
evaporate. Can you feel a little bit cool at your finger

3. Dip the bulb of a thermometer into the same alcohol
and observe the temperature shown.

4. Take out the thermometer and wait until the alcohol
on the thermometer evaporate. Observe the
temperature shown. State either the temperature
increases or decreases ……

The temperature of the
thermometer drops.





A. Apparatus: 250 ml beaker, drinking straw and white tile, alcohol and
water.

1. Pour 100 ml alcohol into a beaker.
2. Touch the outside of the beaker and the

water around the base of the beaker.
Record your observations.
3. Blow air repeatedly into the alcohol.
4. Touch the outside of the beaker. Record
your observations.

Discussion

1. What happens to the alcohol when air is
blown into it?

Alcohol evaporates

2. Compare the level of coldness of the beaker 3. State the effect of
before and after air is blown into the alcohol. evaporation.
Explain your answer.
Evaporation causes
The beaker is colder after the air is cooling to the
surrounding.
blown into the alcohol. When the alcohol

evaporates, its molecules absorb

specific latent heat of vaporisation from

the surrounding in order to break the

bond between molecules. The

surrounding loses heat.

Activity 6 Aim: Applications of specific Latent Heat in everyday life

1. Explain how drinks can be cooled by
adding in several cubes of ices.

• When ice melt,
• it absorbs a large amount of

heat from the drink.
• The drink lost heat,
• so its temperature is lowered.

2. The freshness of fish and meat can
be maintained by placing them in
contact with ice. Explain.

• Ice has larger specific latent heat,
• so it absorbs large amount of heat

from the fish as it melts.
• The temperature of fish decreases.
• Thus the fish can be kept at low

temperature for a longer time